This article conceptualizes the diffusion of user innovations from a service ecosystem perspective. With the focus on sustainable innovations, the service ecosystem is evaluated, along with other systemic innovation concepts, as a possible theoretical basis for explaining the first adoption and diffusion of user innovations. It is proposed that an ecosystem perspective contributes three assumptions that help to better understand the (non)diffusion of sustainability-oriented user innovations: (1) innovation diffusion is a multi-level and -actor phenomenon; (2) an actor-to-actor orientation integrates user innovators into the ecosystem; (3) the service perspective defines innovation diffusion as an evolving co-created process. The assumptions are translated into policy implications and future research requirements for moving towards an innovation infrastructure that considers the role and contribution of users in sustainable innovation.